Advances in plasma processing have facilitated growth in the semiconductor industry. During plasma processing, a semiconductor manufacturer may employ a recipe to etch and/or deposit material on a substrate. The recipe may include a plurality of parameters including, for example, the level of RF power, the gas, the temperature, the pressure, the gas flow rate, and the likes. Each of the parameters of the recipe works together to produce a quality device (e.g., MEMs, etc.). Thus, inaccurate parameters may result in substandard device and/or defective device.
To minimize inaccuracy, the various components that provide the parameters may have to be monitored and/or verified. The flow rate of gas is one such parameter that may have to be verified. During substrate processing, the amount of process gas furnished to the reaction chamber is generally carefully controlled. The indicated gas flow rate (i.e., process gas flow rate) is commonly controlled by a mass flow controller (MFC). Consider the situation wherein, for example a critical process step requires allow rate of 40 standard cubic centimeters (sccm). A process engineer may enter the flow rate in the process recipe and apply the recipe into the plasma tool from a user interface. In entering the recipe flow rate, the process engineer is assuming that the mass flow controller (MFC) will be flowing gas into the reaction chamber at the desired rate. However, the actual flow rate of the gas may vary from the indicated flow rate of the MFC. As discussed herein, an indicated flow rate refers to the flow rate that is shown as the MFC flow rate that is displayed on the plasma tool's user interface.
The accuracy of the indicated flow rate may be dependent upon the accuracy of the MFC. During the manufacture of the MFC, one or more verification test may be performed on the MFC to validate that the gas flow rate control provided by the MFC is within established MFC design specification tolerances. The MFC verification is usually performed in a controlled laboratory environment using an inert gas such as N2 gas. To translate the verification results into corresponding results for other gases (which may be employed in actual production environment), conversion factors may be applied. However, the translated corresponding results may have errors since the conversion factors have an inherent level of uncertainty.
Over time, the MFC performance may degrade resulting in a flow rate inaccuracy. In other words, the indicated flow rate of the MFC and may be outside of the design specification tolerance for the MFC due to calibration drift, zero drift, or gas-calibration error and the MFC may have to be recalibrated or replaced.
A flow verification method is required to determine the percentage of error of the MFC flow rate so that a flow correction can be made to correct the inaccuracy in the gas delivery system. One method that has been employed to validate the indicated flow rate of the MFC is the rate of rise (ROR) procedure. With the ROR procedure, a reaction chamber volume is filled and the pressure rate of rise of the gas is measured. With the ROR method, an actual flow rate for the gas may be determined.
The ROR procedure is a lengthy process which may take about 10 or more hours. The long length time period may be due to the large reaction chamber volume (e.g., up to 60 liters. Other factors include a plurality of gas lines and a plurality of gas boxes in the plasma tool and elevated operating temperatures of certain reaction chambers
In addition to the ROR procedure being a lengthy process, the ROR procedure may also suffered from inaccuracy in matching process results from chamber to chamber. In an example, the volume may vary between chambers of the same size due to manufacturing tolerance of chamber components. In an example, large temperature difference in the chamber may result in a change in volume. Thus, the ROR procedure is a cumbersome method that may introduce longer time duration due to elevated reaction chamber operation temperature.
Also, the ROR procedure may require the plasma tool to be cooled down before the ROR procedure may be performed. The cooling down period may be about 2 or more hours, which represents additional time the reaction chamber is not available for processing wafers. As a result, the ROR procedure may contribute to cost of ownership without really providing a true method for validating the indicated flow rate of the MFC.
Another method that may be employed to verify the indicated flow rate of the MFC includes utilizing a small external ROR chamber or a flow measurement standard (e.g., Molbloc) instead of the actual reaction chamber. With the external flow measurement device method, the external device may be employed as a testing device which may be directly connected lo the MFC to test the flow rate of a gas. Thus, the external device may be employed as a flow verification device.
By employing the external device, a plurality of pressure sensing manometers may be required to accurately measure pressure measurements covering the flow rate of semiconductor manufacturing equipment from 1 sccm to 10,000 sccm. To minimize the time duration of each pressure measurement, a plurality of chamber volumes may have to be designed into the small chamber ROR device. In addition, by employing the smaller chamber ROR device, the time period for filling up the chamber is reduced and the temperature impact on the chamber may also be minimized. However, only inert gases may be tested in the smaller chamber. Thus, real gases that may be employed in etching (e.g., etchant gases) are not tested. As a result, the eternal How measurement device method is unable to test for the effect on flow rate due lo the compressibility of the gases In addition, the smaller chamber ROR device usually requires the utilization of a separate proprietary computer system, thereby not providing an integrated solution with the plasma processing system.